System and Method for Leaching a Metal from a Base Mineral Rock

ABSTRACT

A system and method for leaching a metal from a base mineral rock includes a closed reactor vessel into which a slurry containing the ground base mineral rock is fed. The interior space of the vessel is then pressurized via an oxygen supply to a pressure P. The slurry is then agitated under pressure P and within the closed reactor vessel for a time “t”. A blower or compressor re-circulates the oxygen through the slurry. The required pressure P and time “t” are effective for obtaining a metal recovery of up to 98% of the total metal available in the base mineral rock. Tests demonstrate that a time “t” in a range of 8 to 14 minutes and a pressure P of 10 to 50 psi is effective for obtaining the desired recovery amount, depending on the concentration of attacking agent used.

BACKGROUND OF THE INVENTION

This invention relates generally to systems and methods employed in themining industry to extract certain metals from their original naturalmineral state and, more specifically, to systems and methods that employa leaching process.

In nature, the leaching of metals from a mineral state occurs over timethrough the action of water, air, temperature and other environmentalforces. In the mining industry, this natural process is accelerated toreduce the time required to obtain the metal and increase the amount ofmetal recovered. Leaching operations, therefore, involve the use ofchemicals to extract the metal from the mineral. Depending on suchfactors as the type of mineral, the metal to be extracted, theconcentration of the metal in the mineral, the presence of other metals,and the chemicals used, the leaching process might take as little as afew hours or might take days and even months.

Leaching operations in the gold mining industry take place in large opentanks into which a slurry is pumped that contains the mineral along witha cyanide solution and reagents. The slurry is then agitated bypropeller blades for the amount of time required for 90 to 95% of thegold contained within the mineral to leach into the slurry. The amountof time that the slurry is retained and agitated in the tank variesdepending on such factors as the type of mineral, the granularity of theground mineral, the chemicals used and the type of agitation employed.On average, however, the amount of time required using the currenttechnology takes an average of 12 hours and the extraction of gold inthe best of cases only reaches 90 to 95% of that available in themineral. A need exists, therefore, for a system and method that reducesthe processing time yet increases the yield.

SUMMARY OF THE INVENTION

A system and method for leaching gold, silver or other metals from abase mineral rock includes a closed reactor vessel into which a slurrycontaining the ground base mineral rock, a reagent, and an attackingagent is fed. The rock is preferably ground so that at least 90% of therock is under 80 mesh in size. The attacking agent may have aconcentration in the range of 3 to 10 kg/m3 of attacking solution andmay be a cyanide solution.

The interior space of the vessel is then pressurized via an oxygensupply to a pressure P. The slurry—which may be in a range of 20% to 50%solids by weight and have a pH in the range of 10.5 to 11.5—is thenagitated under pressure P and within the closed reactor vessel for atime “t”. The temperature of the slurry may be in the range of 20° to30° C.

Agitation preferably occurs by way of re-circulating the oxygen withinthe vessel and through the slurry. The agitator preferably includes atleast one blower or compressor and re-circulating piping incommunication with the slurry. Jets may also be employed to enhance theaeration effects within the slurry.

The required pressure P and time “t” are effective for obtaining a metalrecovery of up to 98% of the total metal available in the base mineralrock. Tests demonstrate that a time “t” in a range of 8 to 14 minutesand a pressure P of 10 to 50 psi is effective for obtaining the desiredrecovery amount (depending on the concentration of attacking agent).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of a pressure leaching reactor configured for use in abatch processing operation. A slurry containing a ground base mineralrock, an attacking agent and a reagent is fed into a closed pressurevessel. The vessel is then closed and oxygen is introduced to pressurizethe vessel. While under pressure, the slurry is constantly agitated by ablower or compressor that re-circulates the oxygen and passes the oxygenthrough the slurry.

FIG. 2 is a view of the pressure leaching reactor configured for use ina continuous processing operation. The slurry is agitated under pressureby blowers or compressors that re-circulate the oxygen through theslurry via jets.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments will now be described in reference to the drawingsand the following element numbers:

10 System for leaching metal 12 Slurry source/tank 14 Oxygen source 20Reactor 22 Upper portion 24 Top end 26 Lower portion 27 Baffles 28Bottom end 30 Slurry inlet 32 Oxygen inlet 34 Pressure gauge 36 Outlet38 Discharge nozzle 40 Sight glass 42 Level float 50 Piping arrangement52 Slurry pump 60 Agitator 62 Blower or compressor 64 Re-circulatingpiping 66 Inlet end 68 Outlet end 70 Jet

A system and method for leaching a metal from a base mineral rockemploys an attacking agent mixed with the ground base mineral rock andoxygen that re-circulates through and agitates this slurry mix while theslurry is under a pressure above ambient pressure. Referring now to FIG.1, a system 10 adapted for batch processing includes a reactor 20, apiping arrangement 50 and an agitator 60. Reactor 20 is preferably avertical pressure vessel having a tubular upper portion 22 with top end24 and a conical-shaped lower portion 26 with bottom end 28. A slurryinlet 30 with valve and an oxygen inlet 32 with valve and optionalpressure gauge 34 are located on upper portion 32. Each valve on inlet30 and 32 may be manually or automatically controlled. Oxygen inlet 32is in communication with an oxygen source 14 that provides pure orsubstantially pure oxygen to reactor 20. An outlet 36 with valve islocated at bottom end 28.

Piping arrangement 50 interconnects reactor 20 to a slurry source 12 andprovides a means for transporting the processed slurry to downstreamstorage or processes (not shown) for further processing or analysis. Theslurry entering reactor 20 contains a ground base mineral rock, water,an attacking agent and a reagent in the quantities and proportionsrequired to obtain a slurry of pumping consistency. In the case of goldleaching, the slurry contains the ground base mineral, water, sodiumcyanide, and calcium hydroxide. The proportion of the ingredients willvary accordingly to the type of mineral or the requirements of theoperation. In a preferred embodiment, the slurry is about 40% solids byweight. In other preferred embodiments, the concentration of the slurryis in a range of 20% to 50% solids by weight.

The slurry is fed into reactor 20 through slurry inlet 30 until adesired volume of slurry is contained within the reactor 20. The volumeand level of slurry fed into reactor 20 may be controlled manually andmonitored by a sight glass 40 or controlled and monitored automaticallyvia control means such as a level control float valve 42 (see FIG. 2).Slurry inlet 34 is then closed and oxygen inlet 36 is opened. Pureoxygen or substantially pure oxygen is introduced into reactor 20 untila predetermined pressure P is obtained. In a preferred embodiment, thepressure P is about 40 psi. In other preferred embodiments, pressure Pis in a range of 20 to 50 psi.

Agitator 60 is preferably a forced air system that includes a blower orcompressor 62 located at top end 24 and a re-circulating piping 64having an inlet end 66 in communication with the blower or compressor 62and an outlet end 68 in communication with a bottom portion of theslurry. Agitator 60 re-circulates the oxygen, taking the oxygen residingabove the slurry level and forcing it up through the slurry. Bycontinuously re-circulating the oxygen, agitator 60 continuouslyagitates the slurry for a required amount of residence time. The oxygensupply 14 is preferably maintained at the required pressure P toreplenish any oxygen consumed.

When the required amount of time is accomplished, blower or compressor62 is stopped and any oxygen remaining in the reactor 20 may be removedto storage (not shown) or discarded to atmosphere. When the pressure Pin reactor 20 is substantially equal to atmospheric pressure, outlet 36may be opened to empty reactor 20. The processed slurry may betransported via piping arrangement 60 for storage, further treatment oranalysis.

Referring now to FIG. 2, an alternate embodiment of system 10 includes areactor 20 adapted for continuous processing. Reactor 20 is preferably ahorizontal pressure vessel and may include baffles 27 to isolateportions of the slurry and ensure proper agitation of each portion.Piping arrangement 50 includes a slurry feed pump 52 in communicationwith slurry inlet 30. Jets 70 may be located at the outlet end 68 of there-circulating piping 64.

Similar to the batch operation (see FIG. 1), the ground base rockmineral, the reagents and the cyanide solution are prepared in therequired proportions to obtain a slurry of pumping consistency. Theslurry is pumped to reactor 20 at the required rate and at the requiredpressure while pure or substantially pure oxygen is inserted intoreactor 20 until the pressure P reaches the required pressure. Blower orcompressor 62 re-circulates the oxygen to maintain the slurry inagitation during the required time. The oxygen supply should be kept atthe required pressure to replenish any oxygen consumed. The requiredresidence time of the slurry in reactor 20 may be controlled by adischarge nozzle 38 at outlet 36.

A person of ordinary skill in the art would recognize that reactor 20may be of different types and sizes to accommodate a batch operation,like in a pilot plant or laboratory, or for a continuous operation, likein a commercial treatment plant. Regardless of type or size, reactor 20must have the capability to keep the slurry mixture under pressure andin an agitated state for the required time to allow for the grade ofdilution of the metal required by the process.

Tests using system 10 and carried out by the inventor on ores from ornear Zaruma, Ecuador, achieved recoveries of over 98% with leachingtimes ranging from 8 to 14 minutes, depending on the cyanideconcentration and oxygen pressure. These results were consistent overthe 100 tests carried out by the inventor under the conditions describedbelow. Tests carried out by the inventor on this same type of ore usingopen air tanks and conventional extraction technology demonstrated that,on average, it took 16 hours to obtain at 90% extraction result.

The conditions under which the tests using system 10 were conducted areas follows:

-   -   Test quantity: . . . 14 kg of solids per sample    -   Material grind size: . . . 90% under 80 mesh    -   Cyanide concentration . . . Varied from 3 kg/m³ to 10 kg/m³ of        attacking solution    -   pH of the slurry . . . 10.5 to 11.5    -   Pressure in the reactor: . . . Varied from 10 psi to 50 psi    -   Slurry consistency: . . . Varied from 20% to 50% solids by        weight    -   Oxygen used . . . Industrial type    -   Duration of tests: . . . From 5 minutes to 120 minutes of        pressure agitation    -   Temperature of slurry . . . Ambient, ranging from 20° to 30° C.        Leaching time was found to be proportional to cyanide        concentration and oxygen pressure. Lab analysis of the clear        liquid from the slurry was made using an atomic absorption        spectrometer, with lab analysis of the solids being made by fire        assay.

Due to the prototype construction of reactor 20 for use in the abovetests, any remaining oxygen was discarded to atmosphere. The consumptionof oxygen, therefore, will need to be tested at future. The inventorplans to carry out further tests to find the more efficient parametersof operation for various ore characteristics. A person of ordinary skillin the art would recognize that different types of ores may require, forexample, a different attacking agent or reagent, differentconcentrations of the various ingredients, and different reside timesand pressures.

The tests using system 10 suggest that system 10 significantly reducesthe leaching time and significantly increases the yield (and extractionrate) in comparison to current leaching technologies. System 10 permitsa smaller footprint and reduced power consumption of a leaching sectionin an extraction plant, allows for a coarser grind than the 200 meshcommonly used in milling operations, and reduces the flow time throughand stocks of gold in the extraction plant at any given time. Forexample, because of the reduced residence time and increased extractionrate, gold could be at the foundry within hours rather than days ofleaving the mill. Furthermore, the reduced time to leach the gold doesnot permit leaching of the copper typically present in the ore. Coppertends to cause problems, such as fouling activated carbon, furtherdownstream in the gold recovery process. Last, the density of theprocessed slurry does not affect the recuperation of the gold.

While a system and method for leaching metal has been described with acertain degree of particularity, many changes may be made in the detailsof construction and the arrangement of components without departing fromthe spirit and scope of this disclosure. It is understood that thesystem and method is not limited to the embodiments set forth herein forpurposes of exemplification, but is to be limited only by the scope ofthe attached claims, including the full range of equivalency to whicheach element thereof is entitled.

1. A system for leaching gold, silver or other metals from a basemineral rock, the system comprising: a closed reactor vessel; a slurryincluding a ground base mineral rock, a reagent and an attacking agent;an oxygen supply in communication with an interior space of the closedreactor vessel; and an agitator in communication with the slurry;wherein the interior space of the closed reactor vessel is pressurizedto a pressure “P” and the slurry is agitated under the pressure “P” andwithin the closed reactor vessel for a time “t”, pressure “P” and time“t” being effective for obtaining a metal recovery of up to 98% of thetotal metal available in the base mineral rock.
 2. A system according toclaim 1 further comprising the time “t” being in a range of 8 to 14minutes.
 3. A system according to claim 1 further comprising thepressure P being in a range of 10 to 50 psi.
 4. A system according toclaim 1 further comprising at least 90% of the base mineral rock beingground to a size under 80 mesh.
 5. A system according to claim 1 furthercomprising the slurry being in a range of 20% to 50% solids by weight.6. A system according to claim 1 further comprising a pH of the slurrybeing in a range of 10.5 to 11.5.
 7. A system according to claim 1further comprising a temperature of the slurry being in a range of 20°to 30° C.
 8. A system according to claim 1 further comprising aconcentration of the attacking agent in the slurry being in a range of 3to 10 kg/m³ of attacking solution.
 9. A system according to claim 8further comprising the attacking agent being a cyanide solution.
 10. Asystem according to claim 1 further comprising the agitator including atleast one of a blower and a compressor.
 11. A system according to claim10 further comprising the agitator including at least one of arecirculating piping and a jet.
 12. A method for leaching gold, silveror other metals from a base mineral rock, the method comprising thesteps of: feeding a slurry into a closed reactor vessel, the slurryhaving a ground base mineral rock, a reagent and an attacking agent;pressurizing, an interior space of the closed reactor vessel with oxygento a pressure “P”; maintaining the pressure “P”; and agitating theslurry under pressure “P” and within the closed reactor vessel for atime “t”, pressure “P” and time “t” being effective for obtaining ametal recovery of up to 98% of the total metal available in the basemineral rock.
 13. A method according to claim 12 further comprising thetime “t” being in a range of 8 to 14 minutes.
 14. A method according toclaim 12 further comprising the pressure “P” being in a range of 10 to50 psi.
 15. A method according to claim 12 further comprising at least90% of the base mineral rock being ground to a size under 80 mesh.
 16. Amethod according to claim 12 further comprising the slurry being inrange of 20% to 50% solids by weight.
 17. A method according to claim 12further comprising a pH of the slurry being in a range of 10.5 to 11.5.18. A method according to claim 12 further comprising a temperature ofthe slurry being in a range of 20° to 30° C.
 19. A method according toclaim 12 further comprising a concentration of the attacking agent inthe slurry being in a range of 3 to 10 kg/m³ of attacking solution.